1. Field of the Invention
The present invention relates to an optical head for an optical recording and reproducing apparatus for recording, reproducing and erasing signals with an optical beam by using a magnetooptical effect, the optical head being used for acoustic and video files, document files and external storage device.
2. Description of the Prior Art
Recently, a so-called optical recording and reproducing apparatus using a magnetooptical effect has attracted attention as an apparatus for recording, reproducing and erasing signals repeatedly. An optical head is a basic component of the apparatus which is considered most important technically.
A principle of optical recording, reproducing and erasing is explained here. An information recording medium for an optical recording and reproducing apparatus usually comprises an amorphous film of a rare earth and transition metal system having a vertical anisotropy. A light emitting element is a light source for emitting a linearly polarized light such as a laser diode. On recording, a beam from the light emitting element radiates onto the information recording medium and the temperature increases above the Curie temperature to lose magnetism, and the medium is magnetized along a direction of an applied external magnetic bias field. On reproduction, a direction of polarization of the linearly polarized light is rotated by 0.3.degree.-0.5.degree. left or right according to the direction of magnetization at the surface of the medium due to the magnetooptical effect, and the direction of the rotation is detected to pick up signals. On erasing, a beam from the light source radiates the medium similarly on recording while an external magnetic bias field is applied along a direction opposite to that on recording.
An optical head for the optical recording and reproducing apparatus needs two polarization separation functions. One of them is that the linearly polarized light emitted by the light-emitting element is irradiated efficiently onto an information recording medium while separating a reflecting light including a light with a rotated polarization from the medium from the radiating light to the medium. The other is that the reflected light from the medium is separated into at least two polarization components perpendicular to each other so as to be detected separately in order to detect the direction of the rotation of only 0.3.degree.-0.5.degree..
An optical head having such functions is disclosed for example in U.S. Pat. No. 4,771,414, wherein a polarization beam splitter is used for splitting a light from a light emitting element to an information recording medium and a light reflected from the medium. The polarization beam splitter comprises two prisms adhered to each other and films for separating polarizations are formed at the adhered plane for transmitting P polarization while reflecting S polarization. Then, for example, a linearly polarized light of S polarization is reflected efficiently to radiate an information recording medium, while transmitting P polarization component included in the reflected light.
Further, a polarizer is used in order to split the reflected light into at least two polarization components perpendicular to each other. The polarizer also comprises two quartz prisms adhered to each other, so as to make optical axes thereof 45.degree. each other, so that a polarization plane of the reflected light from the medium has .degree. relative to the optical axis of the plane of the polarizer through which the reflected light enters. Then, the reflected light of P polarization from the medium is split by the polarizer into three beams of P polarization component, S polarization component, and a synthesized component of the P and S polarization components. Then, the S and P polarization components are detected separately and a difference between them is obtained to detect an information signal in the medium.
However, such an optical head is difficult to be made compactly. The beam splitter and the polarizer used for the two beam separation functions both comprise two prisms. Thus, a number of optical components is large, and assemblage processes therefor are needed. Further, volumes thereof are large. Further, a cost of the polarizer is large because quartz prisms are needed. Still further, optical components such as the light emitting element, the polarization separation elements and photosensors need to be located separately, and this requires many adjustment steps.